This invention relates to fluorescent lamps and to methods for the fabrication thereof. More particularly, this invention relates to fluorescent coatings applied to the interior surfaces of the envelopes of such lamps, and to methods for applying such fluorescent coatings. Specifically, this invention relates to aqueous suspensions of phosphor particles, and the improvement in the adherence of fluorescent coatings which can be attained therewith when such suspensions are applied to the interior surface of the envelope of a fluorescent lamp.
Commercially available fluorescent lamps comprise an elongated tubular envelope having a pair of electrodes sealed into the opposite ends thereof. The envelope contains in gaseous atmosphere, which may be a mixture of a rare gas and a metal vapor, such as mercury vapor. The interior surface of the envelope is coated with a finely-divided fluorescent material which is exposed to the discharge between the electrodes, and is excited by ultra-violet radiations emitted by this discharge. The fluorescent coating is usually applied by suspending particulate fluorescent or phosphor material in a suitable binder, flushing the interior of the envelope with the suspension, permitting the excess suspension to drain out of the envelope, and then firing the interior wall of the coated envelope at a temperature which promotes adherence of the coating to the envelope walls and removes, generally by volatilization, the binder material. There results a phosphor film or coating adhered to the inside or interior surfaces of the tubular envelope.
Thus, in the course of manufacture, as well as during operation, of these fluorescent lamps, the phosphors used in these lamps experience a hostile environment. During manufacture, the lamp is baked at temperatures approximating 600.degree.C which can cause serious degradation of some phosphors. While the lamp is operating, the phosphor is in a mercury vapor discharge where it is exposed to high-energy ultra-violet radiation, and is bombarded by electrons and mercury atoms. These factors may be responsible for maintenance losses, i.e., the time-dependent decrease in luminous flux found in all fluorescent lamps.
One method for improving brightness and maintenance is to shield the phosphor from these effects by coating the phosphor particles with a protective film, and numerous attempts have been made to protectively coat the particulate phosphor material by using selected additions to the coating suspension. Such additives include silica, boric anhydride, sodium borate, mixtures of barium and ammonium nitrates, crystalline alkali halides, calcium pyrophosphate, etc. These materials have several disadvantages as lamp phosphor coatings. The polycrystalline additions, by their very nature, cannot be expected to form an impervious film on the phosphors, therefore protection is incomplete. Of the glassy materials utilized, silica requires a very high temperature before it is fluid enough to coat the phosphor particles, whereas the other materials are not chemically compatible with most lamp phosphors and are not very resistant to attack by mercury vapor. In addition, since the phosphors are excited by the 254-nm line of the mercury discharge, the ultra-violet absorption of these glasses must be considered. For example, although boric anhydride, in the pure state, is quite transparent to ultraviolet radiation, additions, such as sodium which can migrate from the glass tubular envelope wall, cause the absorption edge to shift to longer wavelengths which could lead to absorption of the radiation required to excite the phosphor material.
In addition, it is well known that there is a tendency of the fluorescent coatings to chip or flake off the inside surface of the tubular envelope either during the manufacturing operation or during use. Various means have been proposed for improving the adherence of the fluorescent coating to the envelope walls. However, such techniques as have been proposed to date for use in conjunction with aqueous suspensions of the phosphor particles have not been completely satisfactory for a variety of reasons.